What do they put in it?

Lots of Saturdays Mr. Nag and I visit The Book Depot in St. Catharines. We always come out with a carload of books and then we adjourn to a local greasy spoon where I devour a club sandwich and Mr. Nag eats a cheese omelet- same thing every time. We enjoy our meals but, when we leave, we both cough up copious amounts of phlegm. I know this is a disgusting topic but I'm burning to know what the chef (sic) puts in our food that results in temporary (I hope) lung malfunction. Any suggestions? (Not that it will stop us going there.)